Alberto says: The term Third World arose during the Cold War to define countries that remained non-aligned with either NATO (with the United States, Western European nations and their allies representing the First World), or the Communist Bloc (with the Soviet Union,China, Cuba, and their allies representing the Second World).
This terminology provided a way of broadly categorizing the nations of the Earth into three groups based on social, political, cultural and economic divisions.
The Third World was normally seen to include many countries with colonial pasts in Africa, Latin America, Oceania and Asia.
The three worlds as they were separated during the Cold War era, each with its respective allies as of the period between 30 April 1975 (the fall of Saigon) and 23 August 1975 (the communist takeover in Laos).
First World: United States, United Kingdom and their allies.
Second World: Soviet Union, China, and their allies.
Third World: neutral and non-aligned countries.